[cnidocyte]

The idea of quantisation of energy is confusing me. For example in IR, the bond will only absorb energy that is identical to the energy of the bonds bending or stretching vibrations. Why is this and what does the bond do with that new energy?

Then NMR, the proton will only absorb the exact amount of energy required to jump to the higher energy spin state. Why is this? Why doesn't it absorb some energy and make a partial jump towards the higher spin state?

[Borek]

Have you ever played a guitar? String can resonate only at some frequencies, not at any frequency. Same with vibrating molecule, same with electron - it can't have any frequency, it can have only frequency from a given set.

[horsebox]

Never played a guitar no. I'm guessing the frequency of a string can be changed by adjusting the tension but I'd imagine the frequency would change linearly rather than stepwise.

[tamim83]

For example in IR, the bond will only absorb energy that is identical to the energy of the bonds bending or stretching vibrations. Why is this and what does the bond do with that new energy?

The energy of the bond vibrations is quantized.  This means that the bond can only vibrate with certain frequencies (corresponding to discrete energy levels).  When a molecule absorbs a photon of IR radiation it excites a bond vibration to a higher energy state only if the photon's energy is equal to the difference in energy between the initial and final states.  This is what you see in an IR spectrum, different peaks correspond to different bonds absorbing slightly different photons.  IR spectroscopy works because the energy of bond vibration is quantized.  If bonds could vibrate with any frequency, you would not observe peaks in an IR spectrum. 

[Borek]

Never played a guitar no. I'm guessing the frequency of a string can be changed by adjusting the tension but I'd imagine the frequency would change linearly rather than stepwise.

Yes, you can change the tension - but there is no such thing as a modifiable tension in a molecule. It is more like having a given string with a known tension, you can't modify it.

[DrCMS]

Think of it like a giant's staircase you are climbing up (or down).  On your own you can not reach to the next step but if your friend gives you a boost up you can now get to the next step up.  If the boost is a little bit more you can still only get to the next step if the boost is a little bit less you can not even get to the next step.  You can not go up ½ a step only a full step.  If you climb down you can only go down in whole step increments.

[cnidocyte]

I'm slowly getting it. I understand the concept of energy quantisation itself but its all the whys and hows I don't get. I get now that there are no partial absorptions in IR spectroscopy because the bond vibrations can't exist in between these 2 allowable energy states. I don't really understand why there are only certain allowable vibrational frequencies but I better start by learning why the electron of a hydrogen atom can only occupy certain orbitals.